Field of the Invention
This invention relates in general to the field of interferometry and, in particular, to an approach for performing heterodyne interferometry with conventional apparatus fitted with a spectrally controlled light source.
Description of the Prior Art
Spectrally controlled interferometry (“SCI”) is a recently developed interferometric technique that allows implementation of white light interferometry (“WLI”) measurement schemes in common-path interferometers. See U.S. Pat. No. 8,422,026, U.S. Pat. No. 8,810,884 and U.S. Pat. No. 8,675,205, all hereby incorporated by reference. WLI is characterized by the absence of coherent noise because of the light's short coherence length, typically on the order of a few micrometers. On the other hand, dust and other contamination, diffraction on rough surfaces, etc. cause reduced measurement accuracy in conventional high-coherence interferometers such as laser interferometers.
Despite these difficulties, laser interferometry is extremely popular and useful because it allows the use of common-path interferometer designs—a particular class of devices in which most of the errors introduced by the optical system cancel out. This allows the manufacture of less expensive and more accurate instruments. High-coherence interferometry is also described as producing a non-localized interference pattern because the interference of beams occurs over a large volume of space, which is an advantage in setting up the measurement apparatus.
WLI is immune to the problems of laser interferometers but requires careful balancing of the optical path difference between the test and reference arm of the interferometer (OPD) so that interference can take place in the measurement space (i.e., within the coherence length of the light). Such arrangements can be complex and prevent the use of common-path interferometers, therefore forfeiting the above-described advantages. WLI produces localized interference because it is visible only in a limited volume around zero OPD.
SCI successfully combines both approaches and provides the advantages of both common-path interferometry and WLI. SCI produces localized interference in an unbalanced OPD interferometer and thus allows, for example, the use of a Fizeau interferometer in WLI mode, thus eliminating the problem of coherent noise. Therefore, one of the major advantages of SCI is that existing instrumentation can be adapted to its modality of operation by replacing only the laser light source with one capable of proper spectral modulation. Different interferometric techniques can be carried out by manipulating only the spectral properties of such light source. See, for example, the time-multiplexed SCI approach described in copending Ser. No. 14/832,052, hereby incorporated by reference.
Heterodyne interferometry is one of the most precise methods of phase measurement. Its precision can be orders of magnitude better than with conventional phase-shifting interferometry, but it requires laser illumination. Therefore, it is susceptible to the same problems of conventional phase-shifting interferometers; that is, coherent noise and multiple interference. This disclosure relates to the implementation of heterodyne interferometry in a SCI setup that enables ultra-precise measurements with temporally incoherent light, thereby overcoming the problems associated with high-coherence interferometry.